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Aneuploidy is nowadays, painfully enough, associated with an abnormal number of chromosomes, which is the characteristic of genetic development syndromes (e.g. Down syndrome) and deadly cancers. Do we know enough to extend our negative experience to other animals?
In a recent paper, Dr Tom. Pavl. et al. showed, using data gathered from published literature, that aneuploidy is not a technical artefact, but indeed a reality in Oligochaeta (Fig. 1), such as earthworms and leeches, potworms and naidids. The presence of aneuploidy in Oligochaeta is thought-provoking because it contradicts the expectation that sexually reproducing species of Oligochaeta possess equal and constant diploid or polyploid numbers of chromosomes in somatic tissues and constant haploid number of chromosomes in germline.
Figure 1. An example of Oligochaeta: the octochaetid earthworm Dichogaster bolaui (Michaelsen, 1891). This species is widely distributed in the tropics and subtropics. However, it became domicolous in the climatically cooler Middle East and Europe where it lives in flowerpots, sewerage system and toilet bowls. The scale is in millimetres.
P. et al. interpreted the process underlying the observed aneuploidy pattern as similar to the one described by M. P. Walsh in 1954 in a common nightcrawler, Lumbricus terrestris: "Eighteen was the number observed in late diakinesis and the first meiotic metaphase plates. In the spermatogonial divisions, 36 chromosomes were found in the vast majority of metaphase stages. However, there were some first meiotic metaphase plates and late diakinesis figures that showed variations from the number 18. A few very clear metaphase plates showed 17 or 19 bivalents. In some spermatogonial divisions, variations of 34 and 38 were observed. In a few animals, 17 bivalents were seen in some cells while other cells within the same individual showed the usual number".
Aneuploidy was documented in somatic as well as in germ cells. Its presence in somatic tissues could explain the regulation of chromosome expression during the ontogenetic development (known as"chromosome diminution", firstly described by biologist T. Boveri in the parasitic nematode Parascaris). Though, its presence in the germplasm could be seen as a macromutation associated with chromosome and genome rearrangements that produce large saltation phenotype effects. Excitingly, these macromutations might lead us to R. B. Goldschmidt's (geneticist) 'Hopeful Monsters'. "Monsters" were frequently described in Oligochaeta by different authors. Further, if the saltation changes are limited to short periods on the evolutionary scale and are followed by a long period of slow, gradual microevolution, this could conduce to S. Gould's and N. Eldredge's (palaeontologists) punctuated equilibria model of evolution.
Moreover, the data show other properties not expected under the 'artefact model', mainly the significantly lower frequency of odd chromosome numbers compared to the frequency of even chromosome numbers.
The realization that aneuploidy might be common in Oligochaeta should prompt more thorough studies of this phenomenon. Being structurally and functionally far less complex than mammals (they lack, among others, sex chromosomes, Oligochaeta, might be a suitable model to understand the origin and maintenance of aneuploidy.
Tom. Pavl., Senior Researcher, Institute of Evolution, University of Haifa, Haifa, Israel
Tova Cohen, Post-doctorate student, the Institute of Evolution, University of Haifa, Haifa, Israel
ShwetaYadav, Associate Professor, School of Biological Sciences, Dr. H S Gour Central University, Sagar, India
M. Glasstetter, Researcher, Department of Environmental Sciences, Biogeography, University of Basel, Basel, Switzerland
Petr K., Professor, Departments of Chemistry, Physics and Biopharmaceutical Sciences, the University of Illinois at Chicago, Chicago, USA
Oren Pearlson, Lecturer, School of Science and Technology, Tel Hai Academic College, Upper Galilee, Israel
A paper by Dr T. P. et al. about the study appeared recently in "Ecology and Evolutionary Biology".